1. Field of the Invention
This invention relates generally to brushless DC motors and, more particularly, is directed to an improved circuit for driving a brushless DC motor in the forward and reverse directions.
2. Description of the Prior Art
Various types of brushless DC motors are known with such motors generally including at least one pair of magnetic poles, usually formed of permanent magnet north and south pole pieces, secured to the rotor and corresponding energizable coils disposed on the stator with each coil having conductor segments for carrying current in directions which are normal to the magnetic flux generated by the permanent magnet pole pieces. For example, in a 2-pole alternate phase brushless DC motor, a single pair of magnetic poles, comprised of a permanent magnet north pole piece and a permanent magnet south pole piece, may be formed with each pole piece occupying an electrical angle of 180.degree. on the rotor and with two alternately energizable coils also provided on the stator with each coil subtending an arc of 180 electrical degrees. The rotor is thus caused to rotate relative to the stator by a torque produced by the interaction of magnetic flux generated by the pole pieces and the current flow through the coils with the direction of rotation being perpendicular both to the direction of magnetic flux and the direction of current flow.
It is often desirable, in such brushless DC motors, to provide means whereby the rotor can selectively be rotated in either of its forward or reverse rotational directions. Although the direction of rotation of the rotor can be changed by reversing the direction of current flow through the coils, as a practical matter, this cannot be done. That is, in brushless DC motors of the type above-described, switching elements such as transistors are provided for alternately energizing the coils. Because such transistors are uni-directional, that is, only operate in one direction, current cannot be easily made to flow through the coils in the reverse direction.
Conventional DC motors of the brushless type have therefore phase-shifted the current supplied to the coils by 180.degree. in order to provide reverse rotation of the rotor. That is, in the forward direction, current flows through the coils in one direction relative to the north and south magnet pole pieces and in the reverse direction, flows through the coils in the opposite direction in relation to such pole pieces.
In order to provide current energization of the coils, in position sensing device, such as a Hall-effect device, an optical detecting device or the like, is provided to detect the rotational position of the rotor with respect to the stator. The position sensing device produces a position signal which, in conjunction with a control signal corresponding to the desired rotational direction of the rotor, is used to alternately energize the stator coils for rotating the rotor with respect thereto. Such signals may, for example, be applied to a gate circuit comprised of a plurality of NAND gates and inverters which alternately activate the above-described uni-directional switching transistors associated with the coils in correspondence with the rotational position and desired rotational direction of the rotor. However, such gate circuit has proven to be disadvantageous as being too complex in design and operation.